Saturday, February 27, 2016

HOW TO CALCULATED OF THE SOURCES FREQUENCIES



The three sources of frequencies in machines are: generated frequencies, excited frequencies and frequencies caused by electronic phenomena. Generated frequencies sometimes called forcing frequencies are those frequencies actually generated by the machine. Some examples are imbalance, vane pass frequencies (number of vanes times speed), gearmesh frequency number of teeth times speed, various frequencies generated by antifriction bearings, ball passing frequency of the outer race, ball passing frequency of the inner race, ball spin frequency and fundamental train frequency. Generated frequencies are easiest to identify because they can be calculated if the internal geometry and speed of the machine are know.

Some of the calculated frequencies may be present in most machines without indicating a vibration problem. These frequencies at acceptable levels without sidebands, include but not limited to imbalance vane pass frequencies blade pass frequencies and gearmesh frequencies. Other calculated frequencies should not be present in any form at prescribed calibration levels. These frequencies include but not limited to ball pass frequencies of the outer and inner races ball spin frequency, fundamental train frequency. Calculated frequencies should not be modulated with any degree of significance by other frequencies. If any of  these frequencies are generated, a vibration problem exists.

When a rotating unit has a mass balance, it will generate a sine wave that has very little distortion. This signal can be observed in the time domain. The frequency domain spectrum will have a spectral line at one times speed of the unit. For example a 1776 RPM fan that is out of balance will have one spectral line at 29,6 Hz. Most pumps and fans can generated vane or blade pass frequency, which is the number of vanes or blades times the speed of the unit. A high vibration at this frequency could be the result of buildup on the vanes or blades, the vanes or blades hitting something or looseness associated with the rotating unit.  

Gearmesh frequency is normally seen in data taken from a gearbox or gear train. The frequency is the number of teeth on agear times the speed of that gear. For two gears in mesh, the gearmesh frequency will be the same for each gear, thr ratio of the number of teeth to gear speed ia constant. In a gear train, all gears will have the same gearmesh frequency. This vibration caused by teeth rotatingagainst each other. Multiplies and submultiples of gearmesh frequency are sometimes observable in the frequency spectrum and will be discussed later. For illustration consider a gear with 67 teeth is in mesh with a 22-tooth pinion gear. The gear is rotating at 6,4 Hz. Calculated a)what is the gearmesh frequency (GF) b) what is the speed of the pinion gear. The answer are:  
GF = number of teeth x gear speed
a) GF = 67 x 6,4 Hz  = 428,8 Hz
b) Speed of pinion = gear speed / number of teeth
speed of pinion = 428,8 Hz / 22 = 19,5 Hz

Excited frequencies, natural frequencies are the property of the system. Amplified vibration called resonance occurs when a generated frequency is tuned to a natural frequency. Natural frequencies are often referred to as a single frequency. Vibrations are amplified in a band of frequencies around the natural frequencies. The amplitude of the vibration in this band depends on the damping. When refer to natural frequency, often means the center of frequency. Natural frequencies can be excited by harmonic motion if the harmonic motion is within the half power points of the center frequency and contains enough energy.

The half-power points are down 3 dB on their side of the center frequency. The frequency range between these half-power points called the bandwidth of the natural frequency. The half-power point is 0,707 times peak at the center frequency. It is a general rule to stay a least 10% away from each side of the center frequency. If some frequency is within the bandwidth the center of frequency and his frequency contains enough energy to excite the natural frequency, the natural frequency will be present.

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